The semiconductor wafer fabrication process relies heavily on physical inspection processes to ensure product quality. Due to the minute size of the wafer features, highly specialized equipment is required. This equipment typically includes a variety of viewing instruments such as microscopes which permit a technician to accurately magnify and view specific features of the wafer sample.
For various reasons, conventional optical microscopes are ill-fitted for wafer inspection. For example, they are limited in their ability to resolve detail at a level sufficient to enable adequate wafer examination. Additionally, they are unable to achieve the required magnification levels. Furthermore, depth-of-field (i.e., the ability to keep objects at two different depths simultaneously in focus) is restricted, requiring the operator to constantly re-focus the microscope as different areas of the sample are inspected.
These drawbacks are eliminated by using a scanning electron microscope (SEM). Unlike an optical microscope, the SEM utilizes an electron beam to bombard the sample as it sits within a vacuum environment. Due to the characteristics of the electron beam (as opposed to the visible light source used in optical microscopes), resolution and magnification are significantly increased. Additionally, no depth-of-field problems exist with the SEM so surfaces at any depth can be examined without re-focusing. These advantages have made the SEM essential to the wafer inspection process.
Before viewing the wafer sample in the SEM, the wafer must be securely mounted. Typically, the microscope includes a movable base to facilitate specimen mounting. However, an appropriate sample mount is necessary to secure the sample to the base. The sample mount used varies depending on the wafer features to be inspected. For example, sample mounts are known for inspecting the face of the wafer while other mounts permit inspection of wafer edge features. The present invention is addressed to the latter and the remainder of this discussion is directed accordingly.
One apparatus is described by the Applicant herein in a co-pending, commonly assigned application entitled “Wafer Sample Retainer for an Electron Microscope,” filed on Dec. 1, 1997 having Ser. No. 08/980,932.
For semiconductor wafers, inspection of edge features is usually accomplished by securing several wafers together and mounting the sample in a vertical orientation relative to the SEM. The mount typically consists of a vertical member to which one or more wafer specimens are secured using a curable adhesive. Copper tape is then wrapped around the specimens and the mount to secure the sample. While such mounts have proven effective, drawbacks exist. For example, the application and removal of the tape adds additional steps to the inspection process. Additionally, the curable adhesive may require several hours to cure prior to inspection. Furthermore, periodic cleaning of the fixture may be required to remove adhesive residue.
Thus, there are issues concerning increased setup time with current semiconductor sample retaining devices. As wafer fabrication facilities continue to increase production rates, the total number of wafers inspected must also increase. As a result, there is a need for a sample mount that provides quick and effective mounting without the drawbacks inherent with adhesives.